Substance P is a neuropeptide which serves as a neurotransmitter. Little is known of its synthesis or metabolism or the physiologic role that its neurons serve. We have found this neurotransmitter to be involved in the modulation of the dopaminergic system. Other studies in our laboratory have shown that it is apparently metabolized by an enzyme similar to the angiotensin converting enzyme. Preliminary studies have also shown it to be present in adrenal chromaffin granules. A system has been established to measure its rate of synthesis following intraventricular injection of radioactive amino acids.